The printability properties of sized paper are largely determined by the pigment or pigments applied during sizing. The properties of a ready coating slip depend upon the preparation of the individual components of the coating. Thus, the slip must not include agglomerating pigments, unsolved binding agent particles or other impurities, which may cause scratches in the coated sheet. In addition, the slip must have a viscosity which permits the slip to be pumped all the way to the machine and to flow out on the base paper.
The main constituents in a sizing composition which is to be used for sizing paper and cardboard or as paint, are pigment, binding agent and a liquid phase. It is generally also necessary to add a dispersing agent in order to obtain good distribution of the pigment. The viscosity of the composition can thereby often simultaneously be lowered so that a higher dry content can be obtained without a concomitant deterioration in coatability.
For coating paper, cardboard or the like, kaolin, titanium dioxide or chalk have conventionally been employed as pigment. It has also been proposed, however, to use gypsum. Gypsum has not been used in practice, however, probably because it has been considered that gypsum requires a substantially greater amount of dispersing agent.
Conventional binding agents used for sizing compositions include water-soluble substances such as starch and casein, or dispersions of different polymers in water, such as, for example, polyacrylate, polymethacrylate and polyvinyl alcohol. It is also known, however, to use certain cellulose derivatives, such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose and carboxymethyl cellulose. A great number of materials are available for use as dispersing agents. The most commonly used dispersing agents, beyond all comparison, are different types of phosphates, particularly sodium hexametaphosphate. This dispersing agent, however, like several others, has certain disadvantages. For example, the binding force of the binding agent used is often reduced. Furthermore, when these dispersing agents are to be used for the coating of paper which is to be offset printed, they can, in certain cases, give rise to emulsification of the printing ink in the offset water, and an undesired toning of the print may thus result. In view thereof in accordance with the method of the present invention, a dispersing agent is utilized which itself acts substantially as a binding agent, viz. a polysaccharide substituted with carboxyl groups.
Calcium sulphate or gypsum is a natural material found in several countries, for example in the Mediterranean countries. Its usual form is the dihydrate, having the composition CaSO.sub.4.2 H.sub.2 O. Gypsum, however, also exists as a waste product from the manufacture of phosphoric acid, where it often is called "chemical gypsum". The term "calcium sulphate" in connection with the present invention is to be understood as calcium sulphate dihydrate. Due to the abundant supply of gypsum at a relatively low price, it has long been desired to be able to utilize the gypsum as pigment for coating slips. The desire to utilize gypsum for this purpose has not been successfully realized in practice, however, apparently because the special prerequisites which must prevail for obtaining a good result have not been understood.